A single sideband data transmission system which does not require the transmission of a carrier or vestigial sideband for synchronizing the local oscillator is described in U.S. Pat. No. 3,675,131 to Pickholtz. Pickholtz uses a feedback system to generate a phase error signal for correcting the phase of the local oscillator in response to the phase jitter in the single sideband signal. Rather than being derived from a carrier tone or vestigial sideband, the phase error signal is derived from the single sideband input signal itself.
In Pickholtz, the received single sideband signal is demodulated, and its in-phase component and quadrature component is obtained. A detector circuit detects the baseband waveform from the in-phase demodulated component. Another component of the phase error signal is proportioned to the Hilbert transform of the baseband data waveform. This component is obtained by passing the in-phase demodulated component through a detector, for detecting the baseband data waveform, and passing the detected baseband data waveform through a Hilbert transform generator. The phase error signal, derived from the in-phase and quadrature demodulated components and the Hilbert transform of the baseband data waveform is fed back to the local oscillator. The phase of the local oscillator is then adjusted to synchronize with the jittering phase of the received single sideband input signal.
The Pickholtz system is incapable of tracking rapid phase jitter. This will become apparent by considering the above description of the Pickholtz system. As was described, part of the phase error signal fed back to the local oscillator is the Hilbert transform of the baseband data waveform. As is well known to those skilled in the art, the Hilbert transform is equivalent to a 90.degree. phase shift of all frequency components. A 90 .degree. phase shift may only be obtained by an infinite delay. A practical approximation may be obtained after a finite delay, provided the spectral components of the signal do not extend down to DC. Because generation of the Hilbert transform involves a delay, Pickholtz adds an equal delay to the in-phase and quadrature demodulated components in forming the phase error signal.
The presence of these delays in generating the phase error signal prevents the Pickholtz system from tracking rapid phase jitter. Suppose, the received single sideband input wave is subject to rapid phase jitter. The Pickholtz system will generate a phase error signal to adjust the phase of the local oscillator to synchronize with the jittered phase of the input single sideband signal. Since the phase error signal is generated in part by taking the Hilbert transform of the detected baseband data waveform, an appreciable delay will occur before the phase error signal is generated. For rapid phase jitter, the input signal will have jittered to a different value by the time the phase error signal is generated. The phase jitter which was to be corrected has in effect already passed by the time the phase error signal is generated. The delay inherent in the Pickholtz feedback loop will never allow the system to generate the phase error signal in time to compensate for the rapidly changing phase jitter.
Unfortunately, when transmitting data over telephone lines, a phase jitter of a few degrees having a frequency of up to 180 Hz is typically encountered. Because of the inherent delays in the Pickholtz feedback loop, the Pickholtz system is inadequate in maintaining local oscillator synchronization when the input signal undergoes such rapid phase jitter. As a result, single sideband systems still cannot be used for high speed data transmission over poorer telephone lines.